A machine for handling sheet media may comprise storage means, transport means and picking means. The storage means may comprise a number of cassettes, each holding a stack of sheet media. The transport means may comprise a transport mechanism for transporting the sheet media to a dispensing slot. The picking means may comprise a number of pick mechanisms, each associated with a one of the cassettes. Each mechanism may have a movable pick component, such as a pivoting arm, provided with means for holding individual sheets of media. The pick component picks individual sheets from its associated cassette and transfers them to the transport means.
An automated teller machine (ATM) is an example of such a machine. It handles sheet media in the form of currency notes. Each cassette may hold a stack of notes of a particular denomination. One of its roles is to dispense the notes.
Some machines have only a single, main motor driving all of machine's mechanisms. Moreover, some machines have the motor permanently engaged with all of the mechanisms. This means that every mechanism is driven each time a drive demand is placed by any one of the mechanisms. So, for example, in the case of an ATM, during any one dispensing operation, less than all the picking mechanisms may need to be driven, say, because, notes from all the cassettes may not be required; nevertheless, all of the picking mechanisms will be driven, which is inefficient and places a high torque or load demand on the motor. Moreover, the main motor is typically an AC motor, but, because AC mains voltages vary across the world, the motor has to be specific to the country in which the machine will be used, which has inventory implications.
A number of pick mechanisms have been proposed. One pick mechanism uses pneumatic force. An arm has a suction cup at one end. The arm pivots about a support in either of two directions between various positions each defined by an angle of the arm with respect to a reference axis. Each pick operation involves the arm being driven in a sequence of movements from a rest position to a pick position, at which the arm takes hold of the top sheet in the associated stack by applying suction to the cup; to a presentation position, at which the transport mechanism can take hold of the sheet, whereupon the suction is released; and, back to the pick or rest positions. In order for the machine to meet its specifications, particularly as regards pick rate, each movement has to be completed in a minimum time.
The pivoting of the pick mechanism arm may be controlled by a cam which is driven by the main motor. A cam follower, integral with the arm, follows a cam track in the cam. The characteristics of the motion of the arm, such as its position, velocity, acceleration, travel and dwell times, known as the pick profile, are a function of the form of the cam track. For the arm to successfully take hold of any particular type of media, the pick profile must be suitable for that media. The density of media, its substrate, weight, quality and dimensions can all affect the pick profile, which means that pick profiles may be quite different from media to media. In addition, any one sheet in a stack may have an irregularity, such as a hole, which means that a pick profile suitable for every other sheet in the stack may not be suitable for that one sheet. The result may be that the arm may not be able to take hold of that sheet.
The cam track of a cam based pick mechanism control is fixed, which means that the pick profile is fixed. Consequently, if the pick profile of a particular pick mechanism turns out not to be suitable for the media stored in its associated cassette, or any one sheet of media in its cassette, there is little that can be done to vary the control. For example, if the pick profile is unsuitable for one sheet because it has a hole in it and the arm fails to take hold of the sheet at the first attempt, all the pick mechanism can do is retry until, hopefully, it successfully takes hold of the sheet. Ideally, the pick profile would be variable. For instance, a hole in a sheet may necessitate the suction cup extending further into the cassette holding that sheet or maintaining initial contact with the sheet for longer. If the arm cannot take hold of the sheet, operator intervention may be required. If the pick profile is totally unsuitable for the media stored in the associated cassette, the mechanism may have to be changed for a mechanism with a more suitable profile.
It has been proposed, such as, for example, in EP-A-1798694, to replace each main motor-driven cam with an independent stepper motor controlled by an independent stepper motor controller. This decreases the load on the main motor, which continues to drive other mechanisms, and improves efficiency in that each pick mechanism may be operated individually, in isolation from all other mechanisms. In addition, the variable control offered by a stepper motor means that the pick profile of each pick mechanism is independently adaptable. Consequently, by altering the control of a stepper motor, the pick profile of the pick mechanism of which the stepper motor is a component can be adapted to suit the sheet media in the associated cassette without having to change the pick mechanism, or to improve the chances of the pick mechanism taking hold of any one particular sheet, when retrying to take hold of it following a failed first attempt. Also, stepper motors can be very precisely controlled, which is advantageous, and, because they are DC driven, they are not country specific.
However, the high acceleration demands of a pick mechanism and the likelihood of stalling due to the forces associated with picking can be a problem for stepper motors.